Generally, a pressure diffuser useful in the paper pulp industry comprises an elongated generally vertically upstanding vessel which mounts an elongated annular screen for vertical movement within the vessel. Pulp under pressure flows into one end of the vessel and into the annular space between the screen and the exterior vessel wall and through an outlet adjacent the other end of the vessel. As the pressurized pulp traverses the height of the vessel, displacement liquid is introduced into the annular chamber by a plurality of vertically spaced header assemblies. The displacement liquid flows generally radially inwardly through the pulp, treating the pulp, and through the screen into the interior of the vessel furnished with a liquid outlet. The screen is moved in a slow stroke concurrently with the pulp a limited distance of travel and is then returned quickly in a fast stroke to clean the screen by a combined wiping and back flushing action. The different diameters of the upper and lower ends of the screen create filtrate compression and thus back flushing (e.g. see U.S. Pat. No. 4,396,509), during screen movement thus forcing the liquid through the screen holes to back flush the screen.
While such pressure diffusers have been used successfully, a drawback of this type of hydraulic system for moving the screen basket is the extensive load on the hydraulic system. The pressure diffusers have been delivered in different sizes from 1980 and a total of more than 100 units have been installed world wide. Pressure diffusers having a moderate size, i.e. pressure diffuser type TD70 with a washing capacity of above 1000 ADMT/24 h, have a total screen basket weight of about 16 ton, and most recent pressure diffusers for pulp mills of higher production capacity, for example pressure diffuser type TD140 with a washing capacity of above 2000 ADMT/24 h, have a total screen basket weight of about 40 ton. On top of these weights of the screen basket, must also the captured wash filtrate volume be managed, which for these sizes TD70/TD140 amounts to about 4,6 ton and 8,1 ton respectively. The typical design pressure for the hydraulic system for a pressure diffuser type TD70 is between 150 and 250 bar. The installed power rating and energy consumption is thus rather high.